1. Field of the Invention
This invention relates to a power assisted steering system, in particular, for motor vehicles. Such a power assisted steering system contains a servomotor, a servopump, a tank, a control valve that steers the pressure medium delivered by the servopump to the servomotor. Besides, the power assisted steering system contains a control member whose choke cross section can be altered as a function of the vehicle speed. The control valve is made as a four-way valve. It is provided with a controllable feedback device and has a supply connection, a feedback connection, and two working connections for the lines leading to the two working chambers of the servomotor.
2. Description of the Prior Art
Such a power assisted steering system is known from DE-C2-31 22 369. In this power assisted steering system, the hydraulic feedback action can be altered during operation, for example, via the vehicle tachometer as a function of the vehicle speed. That results in a characteristic of the control valve as illustrated, in FIG. 1.
During parking, the control valve works without hydraulic feedback, that is to say, only the mid-centering of the control valve must be overcome as activation moment on the steering wheel. After that, the full hydraulic support sets in to move the steered vehicle wheels (steep characteristic). Once the vehicle begins to roll, then, for the purpose of mechanical mid-centering, one must overcome an additional activation moment that is generated by the hydraulic feedback device, that depends on the pressure, and that rises along with the increasing vehicle speed (flatter characteristic). In the process, as a result of the smaller parking forces, one gets good parking comfort and, due to the feedback of the forces along the steered vehicle wheels to the steering wheel, one gets good properties also in this area when driving fast. The parking forces, however, can never be smaller than the mechanical centering forces. In case of steering corrections when driving straight ahead, however, the reaction forces appearing along the steered vehicle wheels are so small that, for reasons of driving safety at high speeds, these forces should be completely transmitted to the steering wheel. That, again, is possible only with a correspondingly high degree of mechanical mid-centering because the hydraulic support then sets in only later.
In the parking area, the mechanical centering forces should be kept as low as possible. When driving forward fast--on which occasion the servo support should be greatly reduced--one, however, desires strong mechanical mid-centering to get an exact feel of the driving action. These two requirements contradict each other. The design of the mechanical mid-centering arrangement therefore, according to the state of the art, can always be only a compromise between the two demands.
The task of the invention is so to improve the known power assisted steering system that not only the level of the hydraulic feedback but also the onset of servosupport will be controllable.
This problem has already been solved in DE-C2-32 48 252 in that an additional control edge is arranged either between the particular supply control opening and the pertinent working connection or between the supply control opening and the feedback control opening. This additional control edge generates a backup that impacts the feedback chambers with pressure without a corresponding differential pressure being generated in the working cylinder.
That results in a broadening of the characteristic curve in the lower area according to FIG. 2; this, in turn, improves road contact in case of small steering moments, that is to say, in the straight-ahead driving range.
This additional control edge lies in the mainstream and, on account of the narrow gap prevailing there, the stream has a strong laminar portion; that results in a high temperature dependence of the valve system. Besides, to build up the feedback pressure, the control valve must cover a certain distance and that path reduces the effective stiffness of the steering system precisely in the mid-area.